This invention relates in general to force measuring instruments used during non-destructive testing, and more particularly to the measuring of force and moments on wind tunnel models of aircraft or other airborne vehicles.
There are six components of force and moment acting on a wind tunnel model which are of interest to the designer in evaluating the flying qualities of an aircraft. These six components are known by those skilled in the art as lift force, drag force, side force, pitching moment, yawing moment, and rolling moment. By determining the magnitude of these components acting on a scale model in a wind tunnel, certain design parameters can be obtained which will apply to the full scale aircraft.
Prior art strain gage balances have been successfully utilized to measure the forces on wind tunnel models. The moments and forces acting on the model are usually resolved into three components of force and three components of moment by providing different members within the balance that are sensitive only to one or two components. Each of the members carries strain gages which are connected in combinations that form Wheatstone bridge circuits. By appropriately connecting the strain gages, the resulting Wheatstone bridge circuit unbalances can be resolved into readings of the three components of force and three components of moment.
Wind tunnel and non-destructive testing of scale or full size models require the use of six component load measuring devices to measure all the applied loads on a wind tunnel model or test article within 0.3% of maximum load accuracy. These devices are called six-component balances. Prior six-component balances with a two shell structure such as is disclosed in my prior U.S. Pat. Nos. 3,878,713 and 5,201,218 have a core or inner shell, termed the "non-metric" side of the balance, mounted on the sting with the outer shell, the "metric" balance side, mounted on the model. The major weakness of these two shell balances lies in the non-metric inner shell or rod which is necessarily small in diameter and is the major structural load carrying part of the balance. The resulting high stress in the balance webs seriously limit the balance capacity.
In recent years it has become increasingly evident that the accuracy and usefulness of wind tunnel data is directly related to matching full scale Reynolds Numbers in the test facilities. This condition creates significant difficulties for existing six component balances. The extremely high loads generated by the high dynamic pressure require improvements in balance capacity.
All access to the model is by way of the sting support, having the balance attached at the upstream end. The balance must be small enough to fit through the aft end of the model into a cavity within the model. All tubes, hoses, wires and such must compete within the balance for the small cross section of area available within the slim cavity of the models. Achieving higher load carrying capacity by simply enlarging the balance is not acceptable because of the limited maximum diameter available.
Thus, there is a continuing need for improved six-component wind tunnel balances having improved load capacity, the ability to operate at higher Reynolds numbers, decreased cross sectional area to fit within slim models and accurate readings of all six components.